Process and device for evaporating liquids, for example black liquor from cellulose cooking, which contain solid and dissolved substances

ABSTRACT

The present invention relates to a process and a device for evaporating liquids, for example black liquor from cellulose cooking, which contain solid and dissolved substances which are to be concentrated, with liquid being heated to close to boiling temperature in a heat exchanger ( 1 ) of the falling-film type. The process is characterized in that the pressure on the liquid side in the falling-film heat exchanger ( 1 ) is kept so high that boiling does not take place in this heat exchanger and in that the evaporation takes place, by means of flashing, in a separate expansion tank ( 7 ) having a lower steam

TECHNICAL FIELD

[0001] The present invention relates to a process and a device forevaporating liquids, in particular black liquor from cellulose cooking.This liquid contains organic material which has been released from thewood, in particular lignin residues in the form of released lignin,hemicellulose and chemicals which have been added on account of thecooking. The substances are present in the form of solid particles andas dissolved substances. The purpose of the evaporation is toconcentrate the dissolved and solid substances so that they cansubsequently be combusted in what is termed a recovery boiler, if theliquid is a black liquor.

STATE OF THE ART

[0002] It is usual, and has been known for a very long time, toevaporate liquids which contain organic substances or chemicals in orderto be able to combust the residue, and thereby generate heat, and to beable to recover the inorganic chemicals. This technique is particularlywell developed in association with evaporating black liquor fromcellulose cooking. Such an evaporation takes place in several stages inseveral evaporation appliances termed effects, with the most diluteblack liquor being fed into one effect in order to be fed onwards insubsequent effects, with the concentration of the black liquorincreasing as the black liquor is evaporated using steam which isfrequently conducted in a combination of cocurrent and counter-currentdirections relative to the direction taken by the liquor. An example ofa standard coupling of the effects is that the liquor goes in thesequence 456321 if the steam has the sequence 123456. The effects arenormally numbered by the direction of flow of the steam determining thenumber sequence. The steam which is released from the liquor in oneeffect is used for evaporation in the subsequent effect. In the case ofthe highest concentrations, the pressure on both the steam side and theliquor side increases as the concentration increases, which means thatthe temperatures increase the more concentrated the liquor becomes. Thefinished product usually has a dry substance content of 70-80% and cantherefore be combusted in a recovery boiler.

[0003] The appliance which is used for such an evaporation normallyconsists of tall, vertical cylinders which are provided internally witha heating surface consisting of either pipes or lamellae, through whichthe heating steam is caused to flow, with the liquor being caused toflow on the outside as a falling film. In this way, an opencommunication is obtained on the liquor side transversely to thedirection of flow of the liquor. The heating by means of the heatingsurface evaporates the liquor, which is removed at the bottom part ofthe effect, with a part of the liquor being circulated and a part beingconveyed onwards to the next effect.

[0004] In certain evaporators possessing vertical pipes, the liquor iscaused to flow inside the pipes and the steam to surround the pipes,instead of the other way round, with this consequently giving rise to aclosed communication on the liquor side transversely to the direction offlow of the liquor.

[0005] In falling-film evaporations, all the evaporation from the filmbroadly speaking takes place instantaneously. SE-C-504232 presents afalling-film evaporator in which the liquor flows on the outside ofhorizontal pipes, which pipes are heated by aqueous steam. As the liquorfilm falls downwards, it is concentrated, with the emission of liquorvapour, which liquor vapour is led away via an outlet. In this case, theliquor is therefore flashed (decompressed) in the evaporator. A similarfalling-film evaporator is also presented in SE-C-512959, where thisfalling-film evaporator has been provided with an improved system fordistributing the aqueous steam to the pipes. In both these evaporators,the concentration of the liquor is increased successively in thefalling-film evaporator. An improved falling-film evaporator ispresented in U.S. Pat. No. 5,624,531, in which the pipes are arrangedvertically instead, whereupon the black liquor forms a film on theoutside of the pipes. This type of evaporator has been installedextensively in black liquor-evaporation lines and is marketed byKvaerner Pulping AB under the trade name TUBEL™. In this case, too,evaporation from the liquor film takes place continuously and liquorsteam is extracted via the upper outlet at the top.Evaporation-concentrated black liquor is withdrawn at the bottom.

[0006] A type of evaporator is also what is termed a forced-circulationevaporator in which a liquor stream is first heated up in vertical orhorizontal tubular heat exchangers, with the liquor in closedcommunication on the inner side of the tubes, thereby causing thetemperature of the liquor to increase. The actual evaporation only takesplace, by means of flashing, in a subsequent expansion tank having alower steam pressure.

[0007] U.S. Pat. No. 4,857,146 presents a final evaporation stage inwhich the liquor is conducted, in a pressurized state, to a heatexchanger such that no boiling takes place in the heat exchanger. Onlyafter the heating in the heat exchanger is the pressure lowered in adecompression tank (flash tank) to a level below the prevailingsaturation pressure of the liquor. In this case, use is made of aconventional heat exchanger in which the liquor side is completelyfilled.

[0008] U.S. Pat. No. 5,112,441 presents a combination of a conventionalevaporation stage in which evaporation down to 65% dry substance contenttakes place in the evaporation stage (the concentrator), with steambeing blown off from the stage, with this being followed by a subsequentdecompression of the liquor in a flash tank down to a lower, butnevertheless pressurized, level. In this case, evaporation thus takesplace in two stages, firstly in the evaporator and then in a flash tank,but nevertheless while retaining a certain pressure.

[0009] A number of other different constructions and plants which arebased on these principles are well known within the technical field.

The Technical Problem

[0010] Since the liquid which is intended for evaporation contains alarge proportion of solid and dissolved substances, the problem existsthat some of the contents of the liquid can become deposited on thewalls in or on the pipes or lamellae. This so-called incrustationusually arises as a result of the crystallization of dissolved salts onthe heating surfaces. An aggravating factor is when so-called nuclearboiling takes place directly in association with the heating surface,since this boiling can give rise locally to very high concentrations ofsolid or dissolved substances.

[0011] No nuclear boiling, or only very little nuclear boiling, willtake place at the heating surfaces in forced-circulation applianceshaving liquid/liquor sides which are completely filled withliquid/liquor and in which all the evaporation takes place in asubsequent separation space. This a positive factor from the point ofview of incrustation. However, these appliances have a limited capacityto increase the temperature of the liquor when the liquor volumes arerelatively large in relation to the heating surfaces.

[0012] In the falling-film technique, the heating surface and theseparation space are constructed as one unit, which provides a veryefficient construction, which is also characterized by a very efficientuse of the heating surfaces due to its high heat transfer values even ata lower electricity consumption than in the case of theforced-circulation technique. In the falling-film technique, thetemperature of the liquor can be increased rapidly since the liquorvolumes are relatively small in relation to the heating surfaces.However, in a falling-film evaporator, it is not possible to avoidnuclear boiling at the heating surface in association with a relativelyhigh heat load, something which makes this technique more susceptible toincrustation than is the forced-circulation technique.

[0013] That which is most characteristic of the falling-film heatexchanger is that the film constitutes a relatively small part of thevolume on the liquid side, i.e. substantially less than 40% of the totalvolume on the liquid side, normally less than 20% of the volume. Theblack liquor forms a thin film, typically 1-2 mm thick, on the heatingsurfaces. In this way, the volumes of the liquid which have to be heatedcan be reduced relative to the heat-transferring surfaces, resulting inthe film on the heating surfaces being heated rapidly. It is this effectwhich has hitherto been exploited in falling-film evaporators for blackliquor, in which the black liquor undergoes rapid evaporation as thefilm of liquor runs down over the heating surfaces. The remaining partof the total volume on the liquid side is taken up by gas phase(including liquor vapour).

[0014] The consequences of a possible incrustation are very serious bothin the case of forced-circulation evaporation and in the case offalling-film evaporation in conventional tube appliances, both of whichoperate with the liquor in closed communication on the inside of thetubes, since a plugged tube cannot be washed clean during operation bymeans of simply lowering the concentration; instead, the pluggedappliance has to be shut down for mechanical cleaning, resulting in adisturbance of production. In modern falling-film evaporators of thelamella type or of the tube element type (with the falling film on theoutside of the tubes), both of which have open communication on theliquor side, this problem is counteracted by arranging for theconcentration to be lowered during ongoing operation, a procedure whichwill dissolve any incrustation which has formed.

[0015] By means of its construction, with the heating steam inside thetubes, a falling-film evaporator of the tube element type is afavourable mechanical construction, especially at relatively high steampressures.

[0016] While the falling-film technique affords a more efficientevaporation appliance, with lower electrical energy consumption andbetter washing capacity, it has a higher rate of incrustation than doesthe forced-circulation technique, at least at relatively high heatloading.

The Solution

[0017] That which is characteristic of the invention is that use is madeof a falling-film heat exchanger in which, in contrast to knowntechnique, boiling, and thus continuous evaporation in the thin film inthe falling-film heat exchanger, is avoided. By means of pressurizingthe liquor vapour on the liquor side to a pressure which is well abovethe saturation pressure for the liquor at its prevailing temperature,the boiling can be efficiently suppressed and the incrustation problemsavoided.

[0018] According to the present invention, a process for evaporatingliquids, for example black liquor from cellulose cooking, containingsolid and dissolved substances which are to be concentrated has beendeveloped for the purpose of overcoming the above-mentioned problems, inwhich process the liquid in a falling-film heat exchanger of, forexample, the tube of lamella type is heated to the vicinity of theboiling temperature, which process is characterized by thecharacterizing part of Claim 1. The invention also relates to a devicefor evaporating liquids, which device is characterized by thecharacterizing part of Claim 7.

[0019] According to the invention, use if preferably made of afalling-film technique of the tube element type (TUBEL™) or lamellatype, both of which have the falling liquor film on the outside of thetubes or lamellae.

[0020] According to the invention, preference is given to the pressureon the liquid vapour on the liquid side in the falling-film heatexchanger continuously corresponding to, or exceeding, the saturationpressure for the liquid when operation has become established, andcorresponding to, or exceeding, the saturation pressure for the water inthe liquid during black liquor evaporation, and, preferably, to thisbeing more than 0.1 bar above the saturation pressure for the liquid,i.e. the pressure at which the liquid would boil at the highesttemperature prevailing in the falling-film heat exchanger, preferably atleast 0.5 bar above the saturation pressure. There should besubstantially no continuous extraction of liquid vapour from the liquidside. This makes it possible to efficiently suppress nuclear boiling inthe black liquor in the falling-film heat exchanger.

[0021] According to the invention, the liquid is heated using a heatingmedium in the form of steam under pressure. According to a preferredexample of the present invention, the pressure on the heating side inthe falling-film heat exchanger is approx. 3.6 bar and the temperatureis approx. 140° C., while the pressure on the liquid vapour on theliquid side in the falling-film heat exchanger is approx. 2 bar and thetemperature 134° C., with the pressure in the expansion tank beingapprox. 1.5 bar, with the temperature of the liquid being cooled down toapprox. 130° C. during intense flashing.

DESCRIPTION OF THE FIGURES

[0022] The invention will be described in more detail below withreference to the attached figures in which

[0023]FIG. 1 shows an outline diagram of an appliance which is used inassociation with the present invention;

[0024]FIG. 2 shows a more detailed variant of FIG. 1, and

[0025]FIG. 3 shows how the liquid is distributed as a film in thefalling-film heat exchanger.

DETAILED DESCRIPTION

[0026]FIG. 1 shows a falling-film evaporation appliance, i.e. what istermed an effect 1, of conventional construction. This effect canpossess lamellae or pipes in its interior, through or on which theliquid which is intended for evaporation flows. The liquid is fed in viathe pipe 2, and this liquid principally consists of previouslyevaporated, circulating liquid. Some of this liquid, which may be readyfor combusting or which is to be fed to another effect, is withdrawnthrough pipe 3. The circulation is brought about using a pump 4. Theheating medium, i.e. the steam which is to impart to the liquid, whichenters through the pipe 2, the requisite temperature for evaporation, isfed in through the pipe 5. While this steam can be what is termedprimary steam, it can also come from a preceding effect.

[0027] The liquid which has been caused to flow through the effect 1 isremoved at the bottom of the effect through the pipe 6 and fed through apressure reduction valve 10 into the expansion vessel 7, which is at alower pressure than that which corresponds to the pressure at which theliquid coming through the pipe 6 boils. Intensive boiling will thereforetake place in the expansion tank 7, with the liquid being cooled downand vapour being formed, which vapour is removed through the pipe 8. Theliquid which has been concentrated by evaporation is removed from thelower part of the expansion tank 7 and caused to circulate through thepipe 2 or conducted out through the pipe 3. Fresh liquid which is to beconcentrated is fed through the pipe 9 into the expansion vessel 7 andcaused to mix with the liquid which has been concentrated by evaporationin the expansion tank 7.

[0028] The figure only shows the apparatus which is required inaccordance with the present invention diagrammatically. The inventioncan be applied to all the effects in a plant, if so desired, and theseeffects can be of any type whatsoever. That which is important is thatnuclear boiling is prevented in the effect 1 and that the evaporation assuch takes place in the expansion tank 7.

[0029]FIG. 2 shows an advantageous variant in which the components/partswhich have a corresponding function to those shown in FIG. 1 have thesame reference number. In this figure, the falling-film heat exchanger 1is shown with its assembly of vertical tubes 22. A heating medium, i.e.heating steam HS, flows through the tubes, i.e. via the inlet 5, andthence to distribution boxes and out into the tubes 22, and residualsteam CS is ventilated via the outlet 24. Condensate C which has formedis tapped off from the lowest point in the system. The constructionresembles that which is presented in U.S. Pat. No. 5,624,531.

[0030] The liquor which is to be heated on the liquid side is fed in viaan element 21 which is placed at a high level, for distributing theliquid. The distribution element can consist of a box with holes in thebottom from which the liquid drops down over the tube assembly. Theliquid/liquor then forms a falling film of liquid over heating surfaces,which are heated by the heating medium, on the outside of the tubes. Theway in which this film is formed is shown in FIG. 3, in which a numberof tubes, through which heating media SF flow, are seen in crosssection, and in which the film LF is formed on the outside of the tubes,on a liquid side which is otherwise filled with a liquid vapour GF.

[0031] If there is a need to be able to establish the requisite pressureon the liquid vapour on the liquid side already at start-up, means arethen required for supplying pressurized steam. This is obtained via afeed line 30 for pressurizing steam HS, expediently the same steam asused for the heating medium, which steam pressurizes the liquid vapourphase on the liquid side via a pressure control valve 30 a. By means ofsupplying pressure in this way, it is possible to already establish therequisite pressure on the liquid side when starting up from thefalling-film heat exchanger being in a cold state. In certainapplications in which the equipment is run continuously, thepressurization can in this way be dispensed with since the requisitepressure is built up during start-up and, when the operation has becomeestablished (what is termed the steady state), this pressure is thenmaintained due to the heating medium being supplied continuously.

[0032] While the falling-film heat exchanger is not intended to giverise to any boiling of the liquid on the liquid side, temperatures whichare too high can be obtained on the heating side in connection withcertain operational states, a situation which can give rise to pressureswhich are too high. If the liquid contains other non-condensable gaseswhich are more readily volatile than water, for example air or nitrogen,a certain overpressure can also be established. It is possible, via adecompression valve 31 a in an outlet line 31, to ventilate away anypossible overpressure intermittently from the liquid vapour phase on theliquid side to the decompression tank 7, expediently by way of an inlet34 above the liquid level 43 in the tank 7. Due to the equilibriumexisting between water in the liquid vapour phase on the liquid side andthe liquid phase, some water will always be present in the liquid vapourphase. Even if some of the liquid vapour phase has to be ventilated dueto the overpressure which has developed, only a negligibly small part ofthe water will accompany this ventilated vapour, i.e. substantially lessthan 1% of the water content of the integral black liquor, and typicallyof the order of size of a few parts per thousand of the water content inthe integral black liquor. Consequently, substantially more than 97%,typically more than 99%, of the water content in the integral blackliquor should be retained by the falling-film heat exchanger. The liquidwhich is pressurized and heated in the falling-film heat exchanger isconducted to the decompression tank 7, by way of the pipe 6, thepressure reduction valve 10 and an inlet 41, which inlet is arrangedabove the water level 43 formed in the tank, with decompression of theliquid taking place in the gas phase. The pressure reduction valve 10 iscontrolled using a liquid level transmitter 42 such that a certainliquid level is maintained in the falling-film heat exchanger or atleast in the falling-film heat exchanger's outlet pipe 6 for thepressurized and heated liquid. The liquid level is established at alevel which is suitable for ensuring that there is no risk of thepressure in the falling-film heat exchanger blowing out in the tank 7.

[0033] Fresh liquid which is to be evaporated is conducted into thesystem via the decompression tank 7, by way of the pipe 9 and an inlet40 which is arranged above the liquid level 43 which is formed in thetank, with the fresh liquid, which is at a lower temperature and a lowerconcentration, firstly being heated in the gas phase of thedecompression tank by means of direct heat exchange. Expediently, thefresh liquid is conveyed into the gas phase by way of some type ofspray/atomizer (not shown). This results in a first heating of the freshliquid in the gas phase of the tank, with a first degree of incrustationthereby being obtained in a manner which is directed towards the tank 7,and not being obtained in the falling-film heat exchanger. What issubstantially a deliberate formation of any possible incrustations isobtained in the tank 7, thereby considerably reducing the risk ofincrustation formation in the falling-film heat exchanger.

[0034] In order for the present invention to have a particularly goodeffect, it is possible to increase the rate of circulation verysubstantially as compared to what is usual in a conventionalfalling-film appliance, i.e. to what is two to five times the normalrate.

[0035] An example of the actual conditions in the appliance, which canbe a falling-film evaporator with liquid flowing on the outside of thepipes, is that the pressure of the steam which enters through the pipe 5is approx. 3.6 bar, while its temperature is 140° C. In this connection,the pressure on the liquid which is entering through the pipe 2 andwhich is caused to flow on the outside of the vertical pipes in theeffect 1 can be approx. 2 bar while the liquid has a temperature ofapprox. 134° C. This liquid, which is to be partially volatilized in theexpansion tank 7, is admitted into this tank against a steam pressure ofapprox. 1.5 bar, that is a pressure which is approx. 0.5 bar lower thanin the effect 1, with vapour being released rapidly while the liquidcools down to approx. 130° C. The abovementioned approximate figures forpressure and temperature are also intended to include deviations withinthe range +/−10%.

[0036] Apart from saving the costs involved in repeated interruptions toproduction, an advantage of the present invention is that the heatingareas can be made much smaller since there is no need to allow forimpurities. This represents a substantial economy. The invention alsomakes it possible to have fewer sections and fewer circulation rounds,with this too leading to economies as a result of fewer instruments,pumps and control valves. In certain cases, automatic washing valves canalso be dispensed with.

[0037] While the pressure difference between the pressure in theexpansion tank 7 and the liquid side in the effect 1 is expediently lessthan 1 bar, an upper limit is only specified for practical reasons.

[0038] The invention is not limited to the embodiment shown; on thecontrary, it can be varied in different ways within the scope of thepatent claims. For example, the pressurizing 30, 30 a can be omitted ifthe falling-film heat exchanger is essentially run continuously and thestart-up procedure takes place rapidly. The pressure difference betweenthe liquid side and the decompression tank 7 can also be obtained usinga fixed throttle 10, with the difference in height H being establishedfor the purpose of obtaining a suitable pressure difference and thethrottle being adjusted so that no boil-off takes place in the pipe 6.Any overpressure in the falling-film heat exchanger can also beregulated by controlling the flow of the heating medium HS. In thatcase, any possible overpressure valves can only be introduced as anadditional secondary safety measure, where the primary control takesplace by way of the flow of heating medium.

1. Process for evaporating liquids, for example black liquor fromcellulose cooking, which contain solid and dissolved substances whichare to be concentrated, with the liquid being heated in an indirect heatexchanger of the falling-film type having a heating side and a liquidside, where the heating side is entirely filled with a heating medium,preferably aqueous steam, and where the liquid on the liquid side isdistributed over heating surfaces, which are heated by the heatingmedium, in the form of a thin film which occupies less than 40% of thevolume on the liquid side, preferably less than 20% of the volume, andwhere the liquid side is otherwise filled with liquid vapour,characterized in that the pressure on the liquid vapour on the liquidside during established operation is maintained so high that boilingdoes not take place in the liquid on the liquid side and in that theevaporation of the liquid takes place by the pressure on the liquidbeing lowered in a separate expansion tank having a lower pressure. 2.Process according to claim 1, characterized in that the falling-filmheat exchanger is of the lamellar or tubular element type having opencommunication on the liquid side.
 3. Process according to claim 1,characterized in that the falling-film heat exchanger is of the tubularelement type having open communication on the liquid side.
 4. Processaccording to any one of claims 1-3, characterized in that the pressureon the liquid vapour on the liquid side in the falling-film heatexchanger corresponds to, or exceeds, the saturation pressure for theliquid at its highest temperature in the falling-film heat exchanger,i.e. the pressure when the liquid would boil at the prevailingtemperature, preferably more than 0.1 bar above the saturation pressureand even more preferably at least 0.5 bar above the saturation pressure.5. Process according to any one of claims 1-4, characterized in that theliquid is heated using steam under pressure.
 6. Process according to anyone of claims 1-5, characterized in that the pressure on the heatingside in the falling-film heat exchanger is approx. 3.6 bar and thetemperature is approx. 140° C. while the pressure on the liquid side inthe falling-film heat exchanger is approx. 2 bar and the temperature isapprox. 134° C., with the pressure in the expansion tank being approx.1.5 bar and with the temperature of the liquid being cooled down toapprox. 130° C.
 7. Device for evaporating liquids, for example blackliquor from cellulose cooking, which contain solid and dissolvedsubstances which are to be concentrated, comprising an indirect heatexchanger (1) having a heating side and a liquid side, where the heatingside is provided with an inlet (5) and an outlet (24) for heating medium(HS, CS), where the liquid side is provided with an element (21), whichis placed at a high level on the liquid side, for distributing theliquid, which distributing element is arranged to form a falling film(LF) of liquid over heating surfaces (22) which are heated by theheating medium (HS, CS), such that the film (LF) which is formedoccupies less than 40% of the volume on the liquid side, preferably lessthan 20% of the volume, and where the liquid side is provided with anoutlet (6), which is placed at a low level, and means (10; 42) forkeeping this outlet (6) filled with liquid, and where the liquid side isotherwise filled with liquid vapour, characterized in that means (10;42; 30, 30 a) are arranged for pressurizing the liquid vapour on theliquid side, during established operation, to a pressure which exceedsthe saturation pressure for the liquid on the liquid side, with boilingof the liquid being suppressed, and in that a decompression tank (7) isconnected to the outlet (6) for boiling, and thereby evaporating, theliquid by means of decompression via a throttle (10).
 8. Deviceaccording to claim 7, characterized in that the means for pressurizingthe liquid vapour comprise a feed line (30) for heating steam (HS),which is arranged to pressurize, by way of a pressure control valve (30a), the liquid vapour phase on the liquid side.
 9. Device according toclaim 7, characterized in that the means for pressurizing the liquidvapour comprise an outlet line (31) for heating steam, which isarranged, by way of a decompression valve (31 a), to ventilateoverpressure from the liquid vapour phase on the liquid side to thedecompression tank (7).
 10. Device according to claim 7, characterizedin that a means (4) is arranged to maintain a liquid level (43) in thedecompression tank (7), and in that an inlet (41) in the decompressiontank (7), for the liquid from the heat exchanger (1), is arranged abovethe said liquid level (43).
 11. Device according to claim 10,characterized in that an inlet (40), for liquid which is entering thedevice and which is to be evaporated, is arranged in the decompressiontank (7) above the said liquid level (43).